The fastener assembly of the invention finds particular utility as an aircraft structural fastener. However, it will become evident as the description proceeds that the fastener assembly is not limited to such a use, but has general utility wherever structural fasteners are required. The fastener assembly of the invention exhibits the desired characteristic of low bearing deformation which is a function of the malleability of the fastener material. This characteristic is achieved by the composite fastener of the invention since it has a limited amont of highly malleable material in the form of the sleeve, and a major amount of less malleable material in the mandrel.
The net result of the fastener assembly of the invention is the provision of a composite fastener of low bearing deformation. Specifically, in the case of the composite fastener of the invention, there is a limited amount of malleable material adjacent the bore of the hole which receives the fastener. However, the greater portion of the fastener consists of the harder material of the mandrel so as to produce an aggregate fastener of low bearing deformation. The resulting composite fastener of the invention, therefore, has a relatively high yield point, while still conforming at its outer surface closely to fit the hole in the structural members joined thereby, and to introduce the aforesaid interference fit between the fastener and the members.
A further advantage in the use of the composite fastener of the invention is that appropriate selection may be made of the sleeve and mandrel material so as to protect the structure in which the fastener is used against the galvanic corrosion due to contact of dissimilar metals. When the fasteners are used in aircraft, for example, the typical structural materials are aluminum or aluminum alloy. The sleeve of the composite fastener of the invention may then be made of a compatible metal so that there will be virtually no dissimilar metals corrosion between the fastener and the structural materials. Should corrosion occur between the sleeve of the composite fastener and its mandrel, replacements may be made without deterioration of the structure itself.
The concept of the present invention permits, as will be described, a sleeve to be used which has the configuration of a tubular flared rivet, composed, for example, of a relatively soft metal such as aluminum. The concept of the invention also provides an improved means in the form, for example, of a tapered mandrel of relatively hard material for radially stressing the tubular rivet and causing it to flow and fill the hole in the joined structural members, even though the tolerances are not held closely, and even though the tapering of the hole is not uniform. The tapered mandrel, as will be described, firmly retains the hollow sleeve rivet in the hole in the joined structural members by upsetting the end of the hollow rivet remote from its head.
The resulting fastener assembly provides a high shear strength, and it introduces a controlled residual tension stress into the portions of the joined structural members around the hole. This residual tension stress reduces the cyclic stress in the joined members when they are subjected to intermittent tension. This, in turn, reduces the notch effect and the hole effect, and it also reduces the susceptibility of the material of the joined structural members to exhibit fatigue cracks originating at the hole therein. As mentioned above, this residual tension stress is achieved by the fastener of the invention without any concomitant galling or burnishing effect in the bore of the hole which, in the prior art assemblies, has been found to reduce considerably the immunity of the joined structural members to the aforesaid notch and fatigue effects.
The composite fastener of the invention also has a feature in that it may be easily installed by automatic equipment of any appropriate known type. Such equipment, for example, is capable of holding the structural plates to be joined together, of drilling the holes in the plates, of inserting the composite fastener, and of driving the mandrel of the fastener into its installed position.
It may be stated, therefore, that a primary objective of the present invention is to provide a composite fastener which is capable of introducing into the joined structural material a controlled residual tension, without any burnishing or galling effects, which will prolong the fatigue life of the joined material long beyond that achieved by any of the prior art fasteners. The composite fastener of the invention provides a controlled amount of stored energy and associated residual tension stress in the joined material when installed in holes drilled to normal tolerances, so that installation of the composite fastener is simple and straightforward.